1. Field of the Invention
The present invention relates to a method and apparatus for underfilling a semiconductor device. More specifically, the present invention relates to a method and apparatus for uniformly underfilling a bumped or raised semiconductor chip to be essentially void free.
2. Description of the Related Art
Flip-chip and bumped die technology is well known in the art. A flip-chip or bumped die is a semiconductor chip having bumps on the bond pads formed on the active circuit or front side thereof, the bumps being used as electrical and mechanical connectors. The flip-chip is inverted (flipped) and bonded to a substrate by means of the bumps. Several materials are typically used to form the bumps on the die, such as conductive polymers, solder and the like. Where solder balls are employed, the die is often referred to as a Ball Grid Array (BGA). Typically, the solder bumps are reflowed to form a solder joint between the flip-chip and the substrate, forming both electrical and mechanical connections between the flip-chip and substrate. Due to the presence of the bumps on the flip-chip, a gap exists between the substrate to which the flip-chip is attached or bonded and the bottom surface of the flip-chip.
The flip-chip and the substrate typically have different coefficients of thermal expansion, operate at different temperatures, and also have different mechanical properties with differing attendant reactions to mechanical loading and stresses. Because of these differences, stress develops in the joints formed by the bumps between the flip-chip and substrate. Therefore, the bumps must be sufficiently robust to withstand such stressful conditions to maintain the integrity of the joint between the lip-chip and the substrate. To enhance the joint integrity formed by the bumps located between the flip-chip and the substrate, an underfill material comprising a suitable insulating polymer is introduced in the gap between the flip-chip and the substrate. The underfill serves to equalize stress placed on the flip-chip and substrate, to transfer heat from the flip-chip, and to protect the bump connections located between the flip-chip and the substrate from contaminants such as moisture, chemicals, and contaminating ions.
In practice, the underfill material is typically dispensed into the gap between the flip-chip and the substrate by injecting the underfill along two or more sides of the flip-chip with the underfill material flowing, usually by capillary action, to fill the gap. Alternatively, the underfill can be accomplished by backfilling the gap between the flip-chip and the substrate through a hole in the substrate beneath the chip.
However, the traditional methods of underfilling by way of capillary action have disadvantages. As devices shrink, the pads to which the bumps connect shrink commensurately. Accordingly, the bumps employed to attach substrates to the devices, via the pads, are made smaller. Shrinking bumps leave an increasingly smaller gap between the substrate and the mounted device or flip-chip. The shrinking gap between the flip-chip and the substrate inhibits filling the gap in a uniform manner.
Such non-uniform underfilling is particularly prevalent in the areas surrounding the bumps interconnecting the flip-chip to the substrate. When underfilling a flip-chip on a substrate, the underfill material generally is non-uniform in character and may contain bubbles, air pockets, or voids. This non-uniform underfill decreases the ability of the underfill material to protect the interconnections between the flip-chip and the substrate, and environmentally compromises the flip-chip, leading to a reduction in the reliability of the chip.
It would be advantageous to develop a method for performing underfill of semiconductor chips which results in underfill material that is uniform and substantially free of voids or air.